Dehydrogenation of liquid fuel in microchannel catalytic reactor

ABSTRACT

The present invention is an improved process for the storage and delivery of hydrogen by the reversible hydrogenation/dehydrogenation of an organic compound wherein the organic compound is initially in its hydrogenated state. The improvement in the route to generating hydrogen is in the dehydrogenation step and recovery of the dehydrogenated organic compound resides in the following steps: introducing a hydrogenated organic compound to a microchannel reactor incorporating a dehydrogenation catalyst; effecting dehydrogenation of said hydrogenated organic compound under conditions whereby said hydrogenated organic compound is present as a liquid phase; generating a reaction product comprised of a liquid phase dehydrogenated organic compound and gaseous hydrogen; separating the liquid phase dehydrogenated organic compound from gaseous hydrogen; and, recovering the hydrogen and liquid phase dehydrogenated organic compound.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned application having U.S.Ser. No. 10/833,484 and a filing date of Apr. 27, 2004, the subjectmatter of which is incorporated by reference.

BACKGROUND OF THE INVENTION

Hydrogen fueled vehicles, sometimes referred to as the Freedom Car arereceiving considerable interest as part of a plan to reduce the relianceon foreign oil and reduce pollution. There are several current designsof hydrogen cars, with one example being a fuel cell powered vehiclecommonly called an FCV. In the FCV, hydrogen is supplied to a fuel cellwhich produces electricity, which is used to power electric motors thatpropel the vehicle. Another type of hydrogen car is based upon ahydrogen internal combustion engine (HICE). In both designs, hydrogen isthe fuel source with water being generated as the combustion byproduct.

A central issue with respect to both types of hydrogen vehicles, i.e.,the FCV and HICE vehicles, is one of fuel supply. Not only is there alarge infrastructure required for hydrogen dispensation, if oneconsiders all the service stations, production and distributionequipment that are required, but there are issues with respect to fuelhandling and use of the fuel on the vehicle itself. Before there can bea progression to dedicated fuel cell propulsion systems and hydrogeninternal combustion engines, one must foresee a fuel infrastructure.

Two sources of hydrogen for use in hydrogen cars include the reformingof natural gas (fossil fuels) or from water using electrolysis. Oncehydrogen gas is generated it must be stored for subsequent filling ofcars or converted into a liquid fuel. Storage of hydrogen gas requirescompression and transfer to a cylinder storage vessel. And, if thegaseous hydrogen is stored on the vehicle, such storage cylinders areexpensive and they can represent a possible safety hazard in the case ofan accident. Alternatively, hydrogen can be stored under low pressure inmetal hydride canisters, but, at present, hydride canisters are a lotmore expensive than cylinders.

Liquid methanol and other alcohols have been touted as particularlyattractive hydrogen sources because they can be catalytically convertedover a catalyst allowing pure hydrogen to be released on demand. On siteconversion of liquid fuels to gaseous hydrogen overcomes thedisadvantages of gaseous storage. Further, fuels such as methanol, andother alcohols are not overly expensive and there is an infrastructurein place today that allows for handling of liquid fuels. Althoughmethanol and alcohols are suitable as a fuel source, they are consumedin the combustion process. In addition, the byproducts of such catalyticconversion, carbon dioxide and water, cannot easily be converted back toa hydrogen source.

Representative patents illustrating hydrogen storage and use are asfollows:

Hydrogen Generation by Methanol Autothermal Reforming In MicrochannelReactors, Chen, G., et al, American Institute of Chemical Engineers,Spring Meeting, Mar. 30-Apr. 3, 2003 pages 1939-1943 disclose the use ofa microchannel reactor as a means for conducting the endothermicsteam-reforming reaction and exothermic partial oxidation reaction. Bothreactions are carried out in the gas phase.

Scherer, G. W. et al, Int. J. Hydrogen Energy,1999, 24,1157 disclose thepossibility of storing and transporting hydrogen for energy storage viathe catalytic gas phase hydrogenation and the gas phase, hightemperature, dehydrogenation of common aromatic molecules, e.g., benzeneand toluene.

US 2004/0199039 discloses a method for the gas phase dehydrogenation ofhydrocarbons in narrow reaction chambers and integrated reactors.Examples of hydrocarbons for dehydrogenation include propane andisobutane to propylene and isobutene, respectively. Reported in thepublication are articles by Jones, et al, and Besser, et al, whodescribe the gaseous dehydrogenation of cyclohexane in a microreactor.Jones, et al employ a reported feed pressure of 150 kPa and an exitpressure of 1 Pa.

U.S. Pat. No. 6,802,875 discloses a hydrogen supply system for a fuelcell which includes a fuel chamber for storing a fuel such as isopropylalcohol, methanol, benzene, methylcyclohexane, and cyclohexane, acatalytic dehydrogenation reactor, a gas-liquid separation devicewherein byproduct is liquefied and separated from the gaseousdehydrogenation reaction product, and a recovery chamber for thehydrogen and dehydrogenated byproduct.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved process for the storage anddelivery of hydrogen by the reversible hydrogenation/dehydrogenation ofan organic compound wherein the organic compound initially is in itsfully or partially hydrogenated state. It is subsequently catalyticallydehydrogenated and the reaction product comprised of hydrogen andbyproduct dehydrogenated or partially dehydrogenated organic compound isrecovered. The improvement in a route to generating hydrogen viadehydrogenation of the organic compound and recovery of thedehydrogenated or partially dehydrogenated organic compound resides inthe following steps:

introducing a hydrogenated organic compound, typically a hydrogenatedsubstrate which forms a pi-conjugated substrate on dehydrogenation, to amicrochannel reactor incorporating a dehydrogenation catalyst;

effecting dehydrogenation of said hydrogenated organic compound underconditions whereby said hydrogenated organic compound is present in aliquid phase;

generating a reaction product comprised of a liquid phase dehydrogenatedorganic compound and gaseous hydrogen;

separating the liquid phase dehydrogenated organic compound from gaseoushydrogen; and,

recovering the hydrogen and liquid phase dehydrogenated organiccompound.

Significant advantages can be achieved by the practice of the inventionand these include:

an ability to carry out the dehydrogenation of a liquid organic compoundand generate hydrogen at desired delivery pressures;

an ability to carry out dehydrogenation under conditions where theliquid organic fuel source and dehydrogenated liquid organic compoundremain in the liquid phase, thus eliminating the need to liquefy orquench the reaction byproduct;

an ability to employ extended pi-conjugated substrates as a liquidorganic fuel of reduced volatility in both the hydrogenated anddehydrogenated state, thus easing the separation of the releasedhydrogen for subsequent usage;

an ability to carry out dehydrogenation under conditions where there isessentially no entrainment of the hydrogenated organic compound such asthe hydrogenated pi-conjugated substrate fuel source and dehydrogenatedreaction product in the hydrogen product;

an ability to carry out dehydrogenation in small-catalytic reactorssuited for use in motor vehicles;

an ability to generate hydrogen without the need for excessively hightemperatures and pressures and thereby reduce safety concerns; and

an ability to use waste heat from the fuel cell or an IC engine forliberating the hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a dehydrogenation process for producinghydrogen from a liquid fuel while maintaining the liquid fuel anddehydrogenated byproduct in liquid phase.

DETAILED DESCRIPTION OF THE INVENTION

In the process described herein, the fuel source is an organic compoundwhich can be catalytically dehydrogenated forming carbon-carbonunsaturated bonds under liquid phase conditions at modest temperatures.The fuel source can further be described as one that has a low vaporpressure in order to avoid entrainment and loss of liquid fuel in thehydrogen product. Preferably, the vapor pressure is less than 10millimeters mercury at 200 ° C.

In copending application U.S. Ser. No. 10/833,484 having a filing dateof Apr. 27, 2004 which has been incorporated by reference, Pi-conjugated(often written in the literature using the Greek letter π) severalmolecules are suggested as fuel sources of hydrogen which are in theform of liquid organic compounds. These Pi-conjugated substrates arecharacteristically drawn with a sequence of alternating single anddouble bonds. In molecular orbital theory, the classically writtensingle bond between two atoms is referred to as a σ-bond, and arisesfrom a bonding end-on overlap of two dumbbell shaped “p” electronorbitals. It is symmetrical along the molecular axis and contains thetwo bonding electrons. In a “double” bond, there is, in addition, aside-on overlap of two “p” orbitals that are perpendicular to themolecular axis and is described as a pi-bond (or “π-bond”). It also ispopulated by two electrons but these electrons are usually less stronglyheld, and more mobile. The consequence of this is that thesepi-conjugated molecules have a lower overall energy, i.e., they are morestable than if their pi-electrons were confined to or localized on thedouble bonds.

The practical consequence of this additional stability isthat hydrogenstorage and delivery via catalytic hydrogenation/dehydrogenationprocesses are less energy intensive and can be carried out at mildtemperatures and pressures. This is represented by the following. Themost common highly conjugated substrates are the aromatic compounds,benzene and naphthalene. While these can be readily hydrogenated at,e.g., 10-50 atm. at H₂ at ca 150° C. in the presence of appropriatecatalysts, extensive catalytic dehydrogenation of cyclohexane anddecahydronaphthalene (decalin) at atmospheric pressure is only possibleat excessively high temperatures leading to gas phase conditions.

For the purposes of this description regarding suitable organiccompounds suitable as hydrogen fuel sources, “extended pi-conjugatedsubstrates” are defined to include extended polycyclic aromatichydrocarbons, extended pi-conjugated substrates with nitrogenheteroatoms, extended pi-conjugated substrates with heteroatoms otherthan nitrogen, pi-conjugated organic polymers or oligomers, ionicpi-conjugated substrates, pi-conjugated monocyclic substrates withmultiple nitrogen heteroatoms, pi-conjugated substrates with at leastone triple bonded group and selected fractions of coal tar or pitch thathave as major components the above classes of pi-conjugated substrates,or any combination of two or more of the foregoing.

In one embodiment, the pi-conjugated substrates have a standard enthalpychange of hydrogenation, |ΔH_(H2) ^(o)|, to their correspondingsaturated counterparts (e.g., the at least partially hydrogenatedextended pi-conjugated substrates) of less than about 20 kcal/mol H₂ andgenerally less than 15.0 kcal/mol H₂. This value can be determined bycombustion methods or by the ab initio DFT method. For purposes of thehydrogenation/dehydrogenation cycle to store and release hydrogen and tore-hydrogenate the substrate, the extended pi-conjugated substrate mayexist and be cycled between different levels of full or partialhydrogenation and dehydrogenation as to either the individual moleculesor as to the bulk of the substrate, depending upon the degree ofconversion of the hydrogenation and dehydrogenation reactions.

The liquid phase pi-conjugated substrates useful according to thisinvention may also have various ring substituents, such as -n-alkyl,-branched-chain alkyl, -alkoxy, -nitrile, -ether and -polyether, whichmay improve some properties such as reducing the melting temperature ofthe substrate while at the same time not adversely interfering with thehydrogenation/dehydrogenation equilibrium. Preferably, any of suchsubstituent groups would have 12 or less carbons. As discussed below inthe section on “Pi-conjugated Substrates with Multiple NitrogenHeteroatoms” alkyl substituents (and it's expected that also alkoxysubstituents) will actually favorably slightly lower the modulus of theheat of hydrogenation, ΔH_(H2) ^(o).

Extended Pi-Conjugated Substrates

Classes of extended pi-conjugated substrates suitable for the processesof this invention are further and more specifically defined as follows:

Extended Polycyclic Aromatic Hydrocarbons (EPAH). For the purposesherein, “extended polycyclic aromatic hydrocarbons” are defined to bethose molecules having either (1) a polycyclic aromatic hydrocarboncomprising a fused ring system having at least four rings wherein allrings of the fused ring system are represented as 6-membered aromaticsextet structures; or (2) a polycyclic aromatic hydrocarbon of more thantwo rings comprising a six-membered aromatic sextet ring fused with a5-membered ring.

The EPAH molecules represent a particular class of extendedpi-conjugated substrates since their pi electrons are largelydelocalized over the molecule. While, on a thermodynamic basis,generally preferred are the larger molecules (i.e., those withconsiderably more than four rings), the value of the standard enthalpychange of hydrogenation, ΔH_(H2) ^(o), and thus the ease of reversiblehydrogenation can be very dependent on the “external” shape or structureof the EPAH molecule. Fundamentally, the EPAH molecules that have thehighest aromatic resonance stabilization energy will have the lowestmodulus (absolute value) of the standard enthalpy of hydrogenation,ΔH_(H2) ^(o). As is taught by E. Clar in “Polycyclic Hydrocarbons”Academic Press, 1964, Chapter 6, it is a general principle that thestability of isomers of fused ring substrates increases with the numberof aromatic sextets. For instance anthracene

has one aromatic sextet (conventionally represented by three alternatingsingle and double bonds in a single ring or by an internal circle), asfor benzene, while phenanthrene,

has two aromatic sextets, with the result that phenanthrene is morestable by 4.4 kcal/mol (based on the molecules' relative heats offormation).

For an EPAH of a given number of fused-rings the structural isomer thatis represented with the largest number of aromatic sextets and yetremain liquid at reaction temperatures will be preferred as ahydrogenation/dehydrogenation extended pi-conjugated substrate.Non-limiting examples of polycyclic aromatic hydrocarbons or derivativesthereof particularly useful as a fuel source include pyrene, perylene,coronene, ovalene, picene and rubicene.

EPAH's comprising 5-membered rings are defined to be those moleculescomprising a six-membered aromatic sextet ring fused with a 5-memberedring. Surprisingly, these pi-conjugated substrates comprising 5-memberedrings provide effective reversible hydrogen storage substrates sincethey have a lower modulus of the ΔH^(o) of hydrogenation than thecorresponding conjugated system in a 6-membered ring. The calculated(PM3) ΔH^(o) for hydrogenation of three linear, fused 6-membered rings(anthracene) is −17.1 kcal/mol H₂. Replacing the center 6-membered ringwith a 5-membered ring gives a molecule (fluorene, C₁₃H₁₀)

Non-limiting examples of fused ring structures having a five-memberedring include fluorene, indene and acenanaphthylene.

Extended polycyclic aromatic hydrocarbons can also include structureswherein at least one of such carbon ring structures comprises a ketonegroup in a ring structure and the ring structure with the ketone groupis fused to at least one carbon ring structure which is represented asan aromatic sextet. Introducing a hydrogenable ketone substituent into apolyaromatic substrate with which it is conjugated, acceptable heats andhydrogen storage capacities are achievable. Thus for the pigmentpyranthrone,

having a standard calculated enthalpy of hydrogenation is −14.4 kcal/molH₂.

Extended Pi-conjugated Substrates with Nitrogen Heteroatoms can also beused as a fuel source. Extended pi-conjugated substrates with nitrogenheteroatoms are defined as those N-heterocyclic molecules having (1) afive-membered cyclic unsaturated hydrocarbon containing a nitrogen atomin the five membered aromatic ring; or (2) a six-membered cyclicaromatic hydrocarbon containing a nitrogen atom in the six memberedaromatic ring; wherein the N-heterocyclic molecule is fused to at leastone six-membered aromatic sextet structure which may also contain anitrogen heteroatom.

It has been observed that the overall external “shape” of the moleculecan greatly affect the standard enthalpy of hydrogenation, ΔH^(o). The Nheteroatom polycyclic hydrocarbons that contain the greatest number ofpyridine-like aromatic sextets will be the most preferred structure andhave the lowest modulus of the standard enthalpy of hydrogenationΔH_(H2) ^(o) structures. The incorporation of two N atoms in a sixmembered ring (i.e., replacing carbons) provides an even furtheradvantage, the effect on ΔH_(H2) ^(o) depending on the nitrogens'relative positional substitution pattern. A particularly germane exampleis provided by 1,4,5,8,9,12-hexaazatriphenylene, C₁₈H₆N₆,

and its perhydrogenated derivative, C₁₂H₂₄N₆ system

for which the (DFT calculated) ΔH_(H2) ^(o) of hydrogenation is −11.5kcal/mol H₂ as compared to the (DFT calculated) ΔH_(H2) ^(o) ofhydrogenation of −14.2 kcal/mol H₂ for the corresponding all carbontriphenylene, perhydrotriphenylene system. Another representativeexample is pyrazine[2,3-b]pyrazine:

where the (DFT calculated) of ΔH_(H2) ^(o) of hydrogenation is −12.5kcal/mol H₂.

Pi-conjugated aromatic molecules comprising five membered ringssubstrate classes identified above and particularly where a nitrogenheteroatom is contained in the five membered ring provide the lowestpotential modulus of the ΔH_(H2) ^(o) of hydrogenation of this class ofcompounds and are therefore effective substrates for dehydrogenation ina microchannel reactor under liquid phase conditions according to thisinvention. Non-limiting examples of polycyclic aromatic hydrocarbonswith a nitrogen heteroatom in the five-membered ring fitting this classinclude the N-alkylindoles such as N-methylindole,1-ethyl-2-methylindole; N-alkylcarbazoles such as N-methylcarbazole andN-propylcarbazole; indolocarbazoles such as indolo[2,3-b]carbazole; andindolo[3,2-a]carbazole; and other heterocyclic structure with a nitrogenatom in the 5- and -6-membered rings such asN,N′,N″-trimethyl-6,11-dihydro-5H-diindolo[2,3-a:2′,3′-c]carbazole,1,7-dihydrobenzo[1,2-b:5,4-b′]dipyrrole, and 4H-benzo[def]carbazole.

Extended pi-conjugated substrates with nitrogen heteroatoms can alsocomprise structures having a ketone group in the ring structure, whereinthe ring structure with the ketone group is fused to at least one carbonring structure which is represented as an aromatic sextet. An example ofsuch structure is the molecule flavanthrone, a commercial vat dye,

a polycyclic aromatic that contains both nitrogen heteroatoms and ketogroups in the ring structure, and has a favorable (PM3 calculated)ΔH^(o) of hydrogenation of −13.8 kcal/mol H₂ for the addition of onehydrogen atom to every site including the oxygen atoms.

Extended Pi-conjugated Substrates with Heteroatoms other than Nitrogencan also be used as a fuel source and for purposes of this description“extended pi-conjugated substrates with heteroatoms other than nitrogen”are defined as those molecules having a polycyclic aromatic hydrocarboncomprising a fused ring system having at least two rings wherein atleast two of such rings of the fused ring system are represented assix-membered aromatic sextet structures or a five-membered pentetwherein at least one ring contains a heteroatom other than nitrogen. Anexample of an extended pi-conjugated substrate with an oxygen heteroatomis dibenzofuran, C₁₂H₈O,

for which the (DFT calculated) ΔH_(H2) ^(o) of hydrogenation is −13.5kcal/mol H₂. An example of a extended pi-conjugated substrate with aphosphorous heteroatom is phosphindol-1-ol:

An example of a extended pi-conjugated substrate with a siliconheteroatom is silaindene:

An example of a extended pi-conjugated substrate with a boron heteroatomis borafluorene:

Non-limiting examples of extended pi-conjugated substrates withheteroatoms other than nitrogen include dibenzothiophene,1-methylphosphindole, 1-methoxyphosphindole, dimethylsilaindene, andmethylboraindole.

Pi-conjugated Organic Polymers and Oligomers Containing Heteroatoms canalso be used as a fuel source. For the purposes of this description the,“pi-conjugated organic polymers and oligomers containing heteroatoms”are defined as those molecules comprising at least two repeat units andcontaining at least one ring structure represented as an aromatic sextetof conjugated bonds or a five membered ring structure with two doublebonds and a heteroatom selected from the group consisting of boron,nitrogen, oxygen, silicon, phosphorus and sulfur. Oligomers will usuallybe molecules with 3-12 repeat units. While there are often widevariations in the chemical structure of monomers and, often, theinclusion of heteroatoms (e.g., N, S, O) replacing carbon atoms in thering structure in the monomer units, all of these pi-conjugated polymersand oligomers have the common structural features of chemicalunsaturation and an extended conjugation. Generally, while the moleculeswith sulfur heteroatoms may possess the relative ease ofdehydrogenation, they may be disfavored in fuel cell applicationsbecause of the potential affects of the presence of trace sulfur atoms.

The chemical unsaturation and conjugation inherent in this class ofpolymers and oligomers represents an extended pi-conjugated system, andthus these pi-conjugated polymers and oligomers, particularly those withnitrogen or oxygen heteroatoms replacing carbon atoms in the ringstructure, are a potentially suitable substrate for hydrogenation. Thesepi-conjugated organic polymers and oligomers may comprise repeat unitscontaining at least one aromatic sextet of conjugated bonds or maycomprise repeat units containing five membered ring structures. Aromaticrings and small polyaromatic hydrocarbon (e.g., naphthalene) moietiesare common in these conducting polymers and oligomers, often inconjugation with heteroatoms and/or olefins. For example, aheteroaromatic ladder polymer or oligomer containing repeat units suchas:

which contains a monomer with a naphthalene moiety in conjugation withunsaturated linkages containing nitrogen atoms.

A pi-conjugated polymer or oligomer formed from a derivatised carbazolemonomer repeat unit,

can also be used as a fuel source. Other oligomers that contain5-membered ring structures with nitrogen atoms are also subject of thepresent invention. For example, oligomers of pyrrole such as:

which has four pyrrole monomers terminated by methyl groups has an abinitio DFT calculated ΔH_(H2) ^(o) of hydrogenation of −12.5 kcal/molH₂. Other members of this class of pi-conjugated organic polymers andoligomers which are particularly useful according to this invention asextended pi-conjugated substrates are polyindole, polyaniline,poly(methylcarbazole), and poly(9-vinylcarbazole).

Ionic Pi-conjugated Substrates can also be used as fuel source, i.e., ahydrogen source. These ionic pi-conjugated substrates are defined asthose substrates having pi-conjugated cations and/or anions that containunsaturated ring systems and/or unsaturated linkages between groups.Pi-conjugated systems which contain a secondary amirie function, HNR₂can be readily deprotonated by reaction with a strong base, such aslithium or potassium hydride, to yield the corresponding lithium amideor potassium amide salt. Examples of such systems include carbazole,imidazole and pyrrole and N-lithium carbazole. Non-limiting examples ofionic pi-conjugated substrates include N-lithiocarbazole,N-lithioindole, and N-lithiodiphenylamine and the correspondingN-sodium, N-potassium and N-tetramethylammonium compounds.

Pi-conjugated monocyclic substrates with multiple nitrogen heteroatomsare another form of hydrogen fuel source. For the purposes of thisdescription “pi-conjugated monocyclic substrates with multiple nitrogenheteroatoms” are defined as those molecules having a five-membered orsix-membered aromatic ring having two or more nitrogen atoms in thearomatic ring structure, wherein the aromatic ring is not fused toanother aromatic ring. The pi-conjugated monocyclic substrates withmultiple nitrogen heteroatoms may have alkyl, N-monoalkylamino and N,N-dialkylamino substituents on the ring. A non-limiting example of api-conjugated monocyclic substrates with multiple nitrogen heteroatomsis pyrazine.

Pi-conjugated substrates with triply bonded groups can be used as a fuelsource. For the purposes of this description, “pi-conjugated substrateswith triply bonded groups” are defined as those molecules havingcarbon-carbon and carbon-nitrogen triple bonds. The pi-corijugatedmolecules described thus far comprise atom sequences conventionallywritten as alternating carbon-carbon single, and carbon-carbon doublebonds, i.e., C—C═C—C═C— etc., incorporating, at times, carbon-nitrogendouble bonds, i.e., imino groups as in the sequence, C—C═N—C═C—.

An illustration is provided by 1,4-dicyanobenzene:

which can be reversibly hydrogenated to 1,4-aminomethyl cyclohexane:

The enthalpy for this reaction, ΔH_(H2) ^(o), is −6.4 kcal/mol H₂. Table1a. provides representative extended polycyclic aromatic hydrocarbonsubstrates, some of which can be used as a liquid hydrogen fuel sourceor converted to a liquid by incorporating substituents groups such asalkyl groups on the substrate and relevant property data therefor.Comparative data for benzene (1), naphthalene (2, 3), anthracene (46)and phenanthrene (47). TABLE 1a Substrate ΔH°_(H2) (300 K) ΔH°_(H2) (298K) T_(95%) ° C. T_(95%) ° C. Number Substrate Structure (cal.) (exp.)(cal.) (exp.)  1

−15.6 −16.42 319 318   2^(a)

−15.1 −15.29 244 262   3^(b)

−15.8 −15.91 273 280  6

−14.6 226  7

−13.0 169 22

−13.9 206 26

−52.2 27

−17.9 333 28

−14.4 223 31

−14.1 216 34

−14.2 216 46

−15.8 271 47

−14.8 237^(a)Heat of hydrogenation to form cis/decalin.^(b)Heat of hydrogenation to form the trans-decalin.

Table 1b shows extended pi-conjugated substrates with nitrogenheteroatoms some of which may be liquids or converted to liquids andthus suited as a hydrogen fuel source. Property data are included. TABLE1b Substrate ΔH°_(H2) (300 K) ΔH°_(H2) (298 K) T_(95%) ° C. T_(95%) ° C.Number Substrate Structure (cal.) (exp.) (cal.) (exp.) 4

−13.2 −13.37 248 274 5

−15.2 −14.96 268 262 8

−12.2 153 9

−11.9 164 10

−12.5 182 11

−11.2 117 12

−10.6 96 13

−10.7 87 14

−11.4 131 15

−14.4 225 16

−11.5 124 17

−9.7 66 18

−11.7 132 19

−8.7 27 20

−12.1* −12.4* 128 128 21

−12.4 164 23

−14.2 220 24

−14.8 239 25

−12.5 168 30

−12.2 139 35

−13.8 201 36

−15.1 245 37

−12.5 163 38

−15.2 413 39

−9.9 82 40

−8.8 70 41

−6.4 42

−9.0 43

−10.5 88. 53

−13.5 54

−7.7*Calculated and experimental data, both at 150° C.

Table 1c shows extended pi-conjugated substrates with heteroatoms otherthan nitrogen some of which may be liquids or converted to liquids andthus suited for use as fuels. Property data are included. Comparativedata for diphenylsilanes also are shown. TABLE 1c Substrate ΔH°_(H2)(300 K) ΔH°_(H2) (298 K) T_(95%) ° C. T_(95%) ° C. Number SubstrateStructure (cal.) (exp.) (cal.) (exp.) 29

−10.2 52 32

−13.5 197 33

−16.4 285 44

−15.6 275 45

273 55

−17.0 56

−16.4

Table 1d shows pi-conjugated organic polymers and oligomers some ofwhich may be liquids or converted to liquids and thus suited for use asfuels. Property data are included. Comparative data for phenyleneoligomers also are shown. TABLE 1d Substrate ΔH°_(H2) (300 K) ΔH°_(H2)(298 K) T_(95%) ° C. T_(95%) ° C. Number Substrate Structure (cal.)(exp.) (cal.) (exp.) 52

−12.5 57

−15.1 48

−16.0 298 49

−15.7 50

−15.6 51

−15.8

Sometimes one can convert hydrogenated extended pi-conjugated substrateswhich normally would be solid under reaction conditions to a liquid byutilizing a mixture of two more components. In some cases, mixtures mayform a eutectic mixture. For instance chrysene (1,2-benzophenanthrene,m.p. 250° C.) and phenanthrene, (m.p. 99° C.) are reported to form aeutectic melting at 95.5° C. and for the 3-component system consistingof chrysene, anthracene and carbazole (m.p. 243° C.), a eutectic isobserved at 192° C. (Pascal, Bull. Soc. Chim. Fr. 1921, 648). Theintroduction of n-alkyl, alkyl, alkoxy, ether or polyether groups assubstituents on the ring structures of the polycyclic aromaticmolecules, particularly the use such substituents of varying chainlengths up to about 12 carbon atoms, often can lower their meltingpoints. But, this may be at some cost in “dead eight” and reducedhydrogen capacity. As discussed above, certain substituents, e.g.,nitriles and alkynes, can provide additional hydrogen capacity sinceeach nitrile group can accommodate two molar equivalents of hydrogen.

The dehydrogenation catalysts suited for use in microchannel reactorsgenerally are comprised of finely divided or nanoparticles of metals,and their oxides and hydrides, of Groups 4, 5, 6 and 8, 9, 10 of thePeriodic Table according to the International Union of Pure and AppliedChemistry. Preferred are titanium, zirconium of Group 4; tantalum andniobium of Group 5; molybdenum and tungsten of Group 6; iron, rutheniumof Group 8; cobalt, rhodium and iridium of Group 9; and nickel,palladium and platinum of Group 10 of the Periodic Table according tothe International Union of Pure and Applied Chemistry. Of these the mostpreferred being zirconium, tantalum, rhodium, palladium and platinum, ortheir oxide precursors such as PtO₂ and their mixtures, as appropriate.

These metals may be used as catalysts and catalyst precursors as metals,oxides and hydrides in their finely divided form, as very fine powders,nanoparticles or as skeletal structures such as platinum black or Raneynickel, or well-dispersed on carbon, alumina, silica, zirconia or othermedium or high surface area supports, preferably on carbon or alumina.

Having described candidates for use a source of hydrogen and their useas fuels for vehicles, their conversion for on site use is described. Tofacilitate an understanding of the improved step of dehydrogenation ofthe liquid hydrogen fuel sources described herein, reference is made toFIG. 1. FIG. 1 illustrates the use of three microchannel reactors withserial flow of a liquid fuel through the reactors. This reactor schemeillustrated in the flow diagram has been designed for to provide aconstant volume of hydrogen to be generated within each channel of themicrochannel reactors.

Microchannel reactors, which term is intended by definition to includemonolith reactors, are well suited for the liquid phase dehydrogenationprocess. They offer ability to effect the dehydrogenation of hydrogenfuel sources while obtaining excellent heat transfer and mass transfer.In gas phase dehydrogenation, their main deficiency has been one ofexcessive pressure drop across the microchannel reactor. Compression ofthe gaseous reactants comes at a high cost. However, because, inaccordance with this invention, the feed to the microchannel reactors isa liquid, the ability to pressurize the reactor becomes easy. One canpump the liquid fuel to a desired reaction pressure. Thus, pressure dropdoes not become an insurmountable problem as it is in gas phaseproduction of hydrogen. And, as a benefit of the ability to pressurize,it is easy to generate high-pressure hydrogen as a product of thereaction.

Microchannel reactors and monolith reactors are known in the art. Themicrochannel reactors are characterized as having at least one reactionchannel having a dimension (wall-to-wall, not counting catalyst) of 2.0mm (preferably 1.0 mm) or less, and in some embodiments 50 to 500 μm.The height and/or width of a reaction microchannel is preferably 2 mm orless, and more preferably 1 mm or less. The channel cross section may besquare, rectangular, circular, elliptical, etc. The length of a reactionchannel is parallel to flow through the channel. These walls arepreferably made of a nonreactive material which is durable and has goodthermal conductivity. Most microchannel reactors incorporate adjacentheat transfer microchannels, and in the practice of this invention, suchreactor scheme generally is necessary to provide the heat required forthe endothermic dehydrogenation. Illustrative microchannel reactors areshown in US 2004/0199039 and U.S. Pat. No. 6,488,838 and areincorporated by reference.

Monolith supports which may be catalytically modified and used forcatalytic dehydrogenation are honeycomb structures of long narrowcapillary channels, circular, square or rectangular, whereby thegenerated gas and liquid can co-currently pass through the channels.Typical dimensions for a honeycomb monolith catalytic reactor cell wallspacing range from 1 to 10 mm between the plates. Alternatively, themonolith support may have from 100 to 800, preferably 200 to 600 cellsper squared inch (cpi). Channels or cells may be square, hexagonal,circular, elliptical, etc. in shape.

In a representative dehydrogenation process, a liquid fuel 2, such asN-ethyl carbazole, is pressurized by means of a pump (not shown) to aninitial, preselected reaction pressure, e.g., 1000 psia and deliveredvia manifold 4 to a plurality of reaction chambers 6 within a firstmicrochannel reactor 8. (Overall dehydrogenation pressures may rangefrom 0.2 to 100 atmospheres.) As shown, dehydrogenation catalystparticles are packed within the reactor chambers 6, although, as analternative, the catalyst may be embedded, impregnated or coated ontothe wall surface of reaction chambers 6. The reaction channel 6 may be astraight channel or with internal features such that it offers a largesurface area to volume of the channel.

Heat is supplied to the microchannel reactor by circulating a heatexchange fluid via line 10 through a series of heat exchange channels 12adjacent to reaction chambers 6. The heat exchange fluid may be in theform of a gaseous byproduct of combustion which may be generated in ahybrid vehicle or hydrogen internal combustion engine or it may be aheat exchange fluid employed for removing heat from fuel cell operation.In some cases, where a liquid heat exchange fluid is employed, as forexample heat exchange fluid from a fuel cell, supplemental heat may beadded, by means not shown, through the use of a combustion gas orthermoelectric unit. The heat exchange fluid from a PEM (proton exchangemembrane) fuel cell typically is recovered at a temperature of about 80°C., which may be at the low end of the temperature for dehydrogenation.By the use of combustion gases it is possible to raise the temperatureof the heat exchange fluid to provide the necessary heat input tosupport dehydrogenation of many of the fuel sources. A heat exchangefluid from fuel cells that operate at higher temperatures, e.g., 200° C.from a phosphoric acid fuel cell, may also be employed.

In the embodiment shown, dehydrogenation is carried out in microchannelreactor 8 at a temperature of generally from about 60 to 300° C., atsome pressure of hydrogen. Dehydrogenation is favored by highertemperatures, elevated temperatures; e.g., 200° C. and above may berequired to obtain a desired dehydrogenation reaction rate. Becauseinitial, and partial, dehydrogenation of the liquid fuel source occursquickly, high pressures are desired in the initial phase of the reactionin order to facilitate control of the liquid to gas ratio that may occurnear the exit of the reactor chambers. High gas to liquid ratios inreaction chambers 6 midway to the exit of the reactor chambers can causethe catalyst to dry and, therefore reduce reaction rate. In a favoredoperation, the residence time is controlled such that Taylor flow isimplemented, in those cases where the catalyst is coated onto the wallsurface of the reactor, or trickling or pulsating flow is maintained inthose cases where the catalyst is packed within the reaction chamber.(The pulsing flow regime is described by many references (e.g.Carpentier, J. C. and Favier, M. AlChE J 1975 21 (6) 1213-1218) forconvention reactors and for microchannel reactors by Losey, M. W. et al,Ind. Eng. Chem. Res., 2001, 40, p2555-2562 and is incorporated byreference.) By appropriate control of the gas/liquid ratio, a thin filmof liquid organic compound remains in contact with the catalyst surfaceand facilitates reaction rate and mass transfer of hydrogen from theliquid phase to the gas phase.

After a preselected initial conversion of liquid fuel in microchannelreactor 8 is achieved, e.g. one-third the volume of the hydrogen to begenerated, the reaction product comprised of hydrogen and partiallydehydrogenated liquid fuel is sent by line 14 to gas/liquid or phaseseparator 16. Hydrogen is removed at high pressure as an overhead vialine 18 and a high pressure partially dehydrogenated liquid fuel sourceis removed as a bottoms fraction via line 20. High pressure separationis favored to minimize carry over of unconverted liquid hydrocarbonfuel, which typically has a slightly higher vapor pressure than thedehydrogenated byproduct, and contamination of the hydrogen overhead.Advantageously, then the reaction product need not be quenched and thusrendered liquid in order to effect efficient separation of the partiallydehydrogenated organic compound from the hydrogen and minimize carryoverinto the hydrogenated product. This is a favored feature in contrast tothose dehydrogenation processes which use reactants such as isopropanol,cyclohexane and decalin where the dehydrogenation reaction products arein the gas phase.

The bottoms from gas/liquid separator 16 in line 20 is combined andcharged to reaction chambers 22 in second microchannel reactor 24 at thesame or higher temperature in order to maintain reaction rate. Thecooled heat exchange fluid is removed from heat exchange channels 6 vialine 26 and returned to the fuel cell, if liquid or, if the hydrogenexchange fluid is combustion gas, then it is often vented to theatmosphere via line 28.

On recovery of the bottoms from gas/liquid separator 16, the resultingand partially dehydrogenated liquid fuel may be further reduced inpressure than normally occurs because of the ordinary pressure dropwhich occurs in microchannel reactor. The pressure in secondmicrochannel reactor 24 is preselected based upon design conditions butin general a pressure of from 30 to 200 psia can be employed for N-ethylcarbazole. The temperature of the previously but partiallydehydrogenated liquid fuel in reaction chambers 22 is maintained insecond microchannel reactor. Heat to second microchannel reactor 24 issupplied from heat exchange fluid line 10 via manifold 30 to heatexchange channels 31. The use of a lower operating pressure in secondmicrochannel reactor 24 than employed in the first microchannel reactor8 allows for significant dehydrogenation at the design reactiontemperature. Again conversion is controlled in second microchannelreactor in order to provide for a desirable liquid to gas ratioparticularly as the reaction product approaches the end of the reactionchamber. The reaction product comprised of hydrogen and furtherpartially dehydrogenation is removed via manifold 32 and separated ingas/liquid separator 34. Hydrogen is removed as an overhead fromgas/liquid separator 34 via line 36 and a further dehydrogenated liquidfuel is removed from the bottom of gas/liquid separator 34 via line 38.Heat exchange fluid is withdrawn via line 39 from microchannel reactor24 and returned to heat exchange fluid return in line 28.

The final stage of dehydrogenation is carried out in third microchannelreactor 40. The partially dehydrogenated liquid fuel in line 38 isintroduced as liquid to reaction chambers 42 at the same or highertemperature, based on design. Heat is supplied for the endothermicreaction by heat exchange fluid in line 10 via manifold 44 to heatexchange channels 45. As the dehydrogenation approaches equilibrium infinal microchannel reactor 40, i.e., where the final dehydrogenationreaction is carried out at a pressure at the end of the reactor, at ornear atmospheric and at even less than atmospheric conditions if this isrequired to effect the desired degree of dehydrogenation, it isparticularly important to maintain Taylor flow or pulsating flow as thecase may be. Mass transfer of the hydrogen from the liquid phase to thegas phase at or near atmospheric pressure is quite limited. However, lowhydrogen pressures favor completion of the dehydrogenation reaction.

The reaction product from third microchannel reactor 40 is passed togas/liquid separator 46 via manifold 48 where hydrogen is recovered asan overhead via line 50. The dehydrogenated liquid fuel is recovered asa bottoms fraction from gas/liquid separator 46 via line 52 andultimately is sent to a hydrogenation facility. Then the dehydrogenatedliquid fuel is catalytically hydrogenated and returned for service as aliquid fuel source.

In the event that the hydrogenation product in line 50 contains tracesof organic compounds, these may be removed if desired by passing the gasstream through an adsorbent bed (not shown) or an appropriate separatorfor the trace organic impurity.

Although, the dehydrogenation process has been described employing 3microchannel reactors, other apparatus designs and operating conditionsmay be used and are within the context of the invention. The operationparameters are one of process design. The use of multiple reactors, asdescribed, allows for better control of gas/liquid ratios asdehydrogenation of the liquid fuel occurs in the reaction chambers aswell as providing for optimized pressures in dehydrogenation of thevarious organic fuel sources.

1. In a process for the delivery of hydrogen from an organic compoundcapable of reversible hydrogenation/dehydrogenation wherein the organiccompound is initially in its hydrogenated form and subsequentlycatalytically dehydrogenated under dehydrogenation conditions in areactor forming hydrogen and byproduct dehydrogenated organic compound,the improvement in the dehydrogenation step which comprises: (a)introducing said organic compound capable of reversiblehydrogenation/dehydrogenation in liquid form to a microchannel reactorincorporating a dehydrogenation catalyst; (b) effecting dehydrogenationof said organic compound under liquid phase conditions whereby saidorganic compound and said byproduct dehydrogenated organic compoundremain in the liquid phase; (c) recovering a reaction product comprisedof a byproduct dehydrogenated organic compound and gaseous hydrogen; (d)separating the reaction product comprised of said liquid phasedehydrogenated organic compound and gaseous hydrogen into a gaseoushydrogen fraction and liquid phase byproduct dehydrogenated organiccompound; (e) recovering the gaseous hydrogen; and, (f) recovering theliquid byproduct dehydrogenated organic compound.
 2. The process ofclaim 1 wherein the dehydrogenation in step (b) is carried out at apressure of from 0.2 atmospheres to 100 atmospheres.
 3. The process ofclaim 2 wherein the temperature of dehydrogenation is from 60 to 300° C.4. The process of claim 1 wherein the dehydrogenation is carried out ina plurality of microchannel reactors wherein the pressure in eachsucceeding reactor is less than the prior reactor.
 5. The process ofclaim 1 wherein the vapor pressure of the organic compound is less than10 mm mercury at 220° C.
 6. The process of claim 2 wherein the organiccompound forms a pi-conjugated substrate on dehydrogenation.
 7. Theprocess of claim 4 wherein the volume of hydrogen produced in eachmicrochannel reactor is essentially the same.
 8. The process of claim 2wherein the organic compound is selected from the group consisting ofhydrogenated forms of extended polycyclic aromatic hydrocarbons,extended pi-conjugated substrates with nitrogen heteroatoms, extendedpi-conjugated substrates with heteroatoms other than nitrogen,pi-conjugated organic polymers and oligomers, ionic pi-conjugatedsubstrates, pi-conjugated monocyclic substrates with multiple nitrogenheteroatoms, pi-conjugated substrates with at least one triple bondedgroup, a pitch, and any combination of two or more of the foregoing. 9.The process of claim 2, wherein the organic compound in its hydrogenatedform is an extended polycyclic aromatic hydrocarbon selected from thegroup consisting of pyrene, perylene, coronene, ovalene, picene andrubicene, fluorene, indene and acenanaphthylene, pyranthrone; and anycombination of two or more of the foregoing.
 10. The process of claim 2,wherein the organic compound is selected from the group consisting ofN-methylcarbazole, N-ethylcarbazole, N-n-propylcarbazole andN-iso-propylcarbazole and mixtures thereof.
 11. The process of claim 1wherein the dehydrogenation is carried out in a plurality ofmicrochannel reactors wherein the temperature in each succeeding reactoris higher than the prior reactor.
 12. The process of claim 1 whereinalternate or adjacent channels are provided in the reactor channels foruse in transporting heat transfer fluid.